X-ray imaging devices have previously been used as non-invasive medical analyzing devices. In some cases, such medical imaging is performed by placing a subject's body part near an x-ray sensitive film while exposing the body part and the film to x-rays. The x-rays thus expose a monochromatic image on the film corresponding to the body part. Areas of the film exposed to more x-ray radiation appear darker on the film and areas exposed to less radiation appear lighter. Dense material, such as bone, blocks or absorbs a greater portion of the x-rays as compared to less dense material, and thus generates white regions on the image while less dense tissue surrounding the bone appears gray or black. Films have been developed to improve the resolution and detail of the x-ray image as well as reduce the amount of x-ray radiation exposed to the subject. In an effort to improve such analytic imaging properties, the Computer Tomography (CT) scan, Positron Emission Tomography (PET) scan, Magnetic Resonance Imaging (MRI), as well as other imaging device and techniques have been developed. Many such devices have limitations, however. For example, while image quality may be enhanced compared to x-ray film techniques, a significantly higher dosage of x-rays is delivered to the subject. In some cases, for example, the subject may be exposed to as much as 6000 mRem for a CT scan.
Digital imaging has been developed to employ detectors that are configured to detect x-rays or electromagnetic radiation having wavelengths in the range of about 0.01 nanometers to about 10 nanometers. Typically such detectors include a photodiode and a scintillator. Scintillators function by converting impinging x-rays into photons of visible light. This visible light is emitted by the scintillator and is detected by the photodiode, and thus is converted into an electrical signal that can be digitized and used to create a digital image.
Scintillators when used in combination with a photomultiplier tube (PMT) or an avalanche photodiode (APD) can enhance the detection characteristics of the system. PMTs are detectors that are very sensitive to light in the ultraviolet, visible, and near-infrared regions. PMTs function by multiplying the current produced by light incident on the tube by as much as 100 million times, which can enable individual photons to be detected when the incident flux of light is very low. APDs work by applying a high reverse bias voltage, typically around 200V, to the device. The high voltage creates a strong electrical field that can generate multiple electrons per incident photon on the device, also known as internal current gain. PMTs and APDs present large integration challenges for the equipment manufactures as these devices can require drive voltages in excess of 300V to extract sufficient signal for measurement.
Scintillator devices, however, often suffer from drawbacks such as limited responsivity, limited spectral detectability, and performance degradation due to factors such as the Staebler-Wronski Effect (SWE). In some cases, x-rays passing through a scintillator material can be affected by fluorescence/phosphorescence effects and optical scattering, thus causing spatial blurring and signal to noise limitations that can ultimately degrade image quality, and thus may not be desired for some types of radiography.
Digital imaging device have been developed that exclude the use of scintillators. Amorphous selenium flat-panels, for example, are able to capture and convert x-ray energy directly into electronic signals without the aid of a wavelength converting material such as a scintillator. However, such devices also have limitations, particularly in terms of responsivity, temperature range, longevity, and application use. Limited responsivity and spectral detection can relate directly to degrade image quality. Further, the Staebler-Wronski effect can have a negative impact after several hours of use on devices that include amorphous semiconductor materials. It is understood that the Staebler-Wronski Effect impacts amorphous semiconductor materials by changing the properties of hydrogenated amorphous silicon (a-Si:H) or non-hydrogenated amorphous silicon. The defect density of the a-Si compound increases with light exposure, thus causing an increase in recombination occurrences, and leading to the reduction in incident electromagnetic radiation to electricity conversion efficiency.
Furthermore, amorphous selenium detectors typically have a narrow operating or stagnant temperature range. In order for such devices to work properly, selenium detectors often need to be maintained within a temperature range of between about 5° C. and 30° C.; operating outside this range can have a detrimental effect on the selenium. Selenium can also generate ghost images if x-ray source energies in the range of 180 keV or more are applied. In addition, the lifetime of the selenium flat-panels can be relatively short, in some cases less than 2 years.